probably the difficulty of getting the proper plastic/polymer, I have seen this for years about trying to make polymer handguns, the closest I have ever seen was two halfs of aluminum or the use of airsoft receivers reinforced, great thread on this over at WG

well if it ain't that nylon shit it ain't worth doing in plastic generally speaking--

those hard plastic injection molded lowers just plain suck--

you might be able to do some kind of plastic/metal sandwich --in a home shop --and have a mold for two halves --then bond them together with sheet steel or aluminum reinforcements in certain breakage prone places maybe??